1. Field of the Invention
The invention relates to flexible or programmable, fast and power efficient wired or wireless communication systems and methods based on the orthogonal frequency division multiplexing modulation technique.
2. Description of the Related Art
The idea to use multicarrier methods as a modulation technique is known in the prior art (Chang, R. W. Synthesis of band-limited orthogonal signals for multichannel data transmission, Bell syst. Tech. J., vol.45, pp. 1775-1796, December 1966 and Saltzberg, B. R. Performance of an efficient parallel data transmission system, IEEE Trans. Comm. Technol., vol. COM-15, December 1967). The possible benefits following from multicarrier methods have been mentioned in many articles (Mxc3xauller, T. Brueninghaus, K. and Rohling H. Performance of Coherent OFDM-CDMA for Broadband Mobile Communications, Wireless Personal Communications 2, Kluwer Academis Publishers, 1996, pp. 295-305 and Kaiser, S. OFDM-CDMA versus DS-CDMA: Performance Evaluation for Fading Channels, ICC ""95, pp. 1722-1726). The basic idea of multicarrier methods is to distribute the bits of the data signal to be transmitted over a plurality of carriers. Theoretical publications have been written on this attractive modulation technique (Kalet, The multitone Channel, IEEE Trans. Commun., vol. 37, no. 2, February 1989 Fazel and G. Fettweis, Multi-Carrier Spread-Spectrum, Kluwer Academic Publ., 1997). These articles only consider one aspect of the complete communication system, or they presume assumptions such as perfect synchronisation or a Gaussian channel, by which some of the technically most arduous problems are avoided. First only simple complete multicarrier (MC)-based communication systems were realised. For a large number of carriers, the arrays of sinusoidal generators and coherent demodulators required in a parallel system become unreasonably expensive and complex. Second, the idea to use the Discrete Fourier Transform (DFT) to realise parallel data transmission systems was applied (Weinstein, S. B. and Ebert, P. M. Data transmission by frequency division multiplexing using the discrete Fourier transform, IEEE Trans. Comm. Technol., vol. COM-19, No. 15, October 1971). Thanks to the evolution of Digital Signal Processing and VLSI technology, the actual use of MC modems based on (I)FFT processing (these are often called Discrete Multi Tone (DMT) modems), has become feasible (Bidet, E. Joanblanq, C. and Senn, P. A fast 8K FFT VLSI chip for large OFDM single frequency networks, Proc. Intl. Conf. On HDTV 94, Torino, Italy, October 1994). At the moment, DMT modems have been realised for wired communication media. For example, xDSL-products for copper wire transmission, are commercially available nowadays (Chow, J. S. Tu, J. C. and Cioffi, J. M. A discrete Multitone transceiver system for HDSL applications, IEEEE J. on Selected Areas in Commun., vol. SAC-9, August 1991). Wireless channels and wireless terminals impose however significant different working conditions and requirements on the transceiver modems. For example, the variability of the channel is much higher in wireless applications, and thus special measures accounting for this have to be taken (e.g. an adaptive equaliser is needed). Also, the echo-profile is significantly different on wireless channels and asks for dedicated approach in the modem. On wireless channels, broadcasting systems based on OFDM are very attractive (The COFDM system intended for Digital Audio Broadcasting takes benefit from time, frequency and space diversity, French Contribution to CCIR WP10 and 10-11S, October 1991), and the Digital Audio Broadcasting (DAB) system is already operational. These broadcasting systems involve one-way transmission, and thus separate transmitter and receiver modems have been developed and built in this context (Van de Laar, F. Philips, N. and Huisken, J. Towards the next generation of DAB receivers, EBU Technical Review, No. 272, Summer 1997, pp. 46-59). For broadcasting applications, the transmitter power is high and there is no need to minimise this, in contrast to point-to-point communication applications. Moreover, in the acquisition strategy it can be assumed that there is always a signal present. A null in the signal is used to obtain rough synchronisation. These are conditions that are not valid for point-to-point communications, and as a consequence the difficult acquisition problem should there be handled in a different way.
Aim of Invention
It is the aim of the invention to present an apparatus and a method for receiving and/or transmitting signals using orthogonal frequency multiplexing which can be used but is not limited to conditions for wireless and point-to-point communication. Further said apparatus and said method provides a solution for all critical blocks and implementation issues needed for the realisation of an OFDM-based transceiver. The apparatus and method show a large flexibility, enabling optimisation towards particular communication situations. The resulting apparatus and method are efficient for power consumption and speed, resulting in high data rates.
In a first aspect of the invention an apparatus for signal transmitting and/or receiving preferably used for but not limited to wireless communication and exploiting OFDM as modulating technique is disclosed. Said signal transmitting and/or receiving apparatus using orthogonal frequency division multiplexing comprises of a first frequency domain circuit (105), a transformation circuit (103), a time synchronisation circuit (17) and a second frequency domain circuit (13). Said circuits are interconnected.
Signals, being used in said apparatus, are characterised to be either in a frequency domain representation (104, 107) or in time domain representation (101, 102, 100, 106, 108).
Signals, being used in said apparatus, are characterised to be either non-orthogonal frequency division multiplexing signals or orthogonal frequency division multiplexing signals. With being a non-orthogonal frequency division multiplexing signal is meant that the signal contains at least some non-OFDM symbol samples. Alternatively an orthogonal frequency division multiplexing signal comprising entirely of OFDM-symbol samples. With an OFDM symbol is meant a symbol that is generated by transforming a sequence coming from said second frequency domain circuit (13) and transformed by said transformation circuit (103). This characterisation is only done for signals in time domain representation. Signals can comprise of parts. Such a part can be either an OFDM-signal or a non-OFDM signal. Signals (102, 108) are OFDM signals. Signal (101) comprises at least of a first part, being a non-OFDM signal and a second part, being a OFDM signal. Signals (100, 106) are non-OFDM signals.
In a first embodiment of this first aspect of the invention it is indicated that in the receiving part of the circuit (circuits (17), (103), (105)) the time synchronisation circuit (17) determines control information on that part of its input (101) being a non-orthogonal frequency division multiplexing signal, for instance partly comprising of a Pseudo-Noise sequence. Said control information can be signal level information, carrier offset or timing information. The signal conversions in the time synchronisation circuit (17) comprise at least of carrier offset compensation and guard interval removal.
In a second embodiment of this first aspect of the invention both the receiving part (circuits (17), (103), (105)) and the transmitting part (circuits (13), (103)) of the circuit are presented. The transformation circuit (103) is part of both the transmitting and the receiving part of the circuit.
In a further embodiment of this first aspect of the invention said transformation circuit (103) comprises of a cascade of a fast Fourier transform circuit (15) and a symbol reordering circuit (16), enabling guard interval introduction.
In a further embodiment of this first aspect of the invention said first frequency domain circuit (105) comprises of a cascade of an equalisation circuit (18) and a demapping circuit (19).
In a further embodiment of this first aspect of the invention said time synchronisation circuit comprises of a timing synchronisation circuit (40), a gain or signal level control circuit (39), a carrier offset estimation circuit (41) and a carrier offset compensation circuit (42). The time synchronisation circuit can also remove the guard interval.
In a further embodiment of this first aspect of the invention it is emphasized that the reordering circuit is exploited as a first step for despreading, (for instance) by organising related carriers together to facilitate easy despreading.
In a further embodiment of this first aspect of the invention the flexibility of the circuit is presented. In said circuit, the integer N, characterising the N-point Fast Fourier Transform, the amount of carriers, the length of the guard interval, the spreading factor and spreading code and the amount of zero carriers, are programmable.
In a second aspect of the invention methods for operating an apparatus for receiving and/or transmitting signals are disclosed. Said transmitting and/or receiving apparatus can be said apparatus of the first aspect of the invention although the operating methods are not limited thereto.
In a first embodiment of this second aspect of the invention in said apparatus two data flows are defined and only one of said data flows is active at a time. One data flow (13), (103) is active during transmitting, the other during receiving (17), (103), (105). As such a half-duplex mode of operation is presented. The time synchronisation circuit (17) controls the powering on or off of the transformation circuit (103) and the first frequency domain circuit (105) during receiving.
In a second embodiment of this second aspect of the invention the operation of said symbol reordering circuit (16) is disclosed.
In a third aspect of the invention a method for communicating data between a transmitting apparatus and a receiving apparatus, is disclosed. Said method for communicating data between a transmission circuit and a receiving circuit using orthogonal frequency division multiplexing comprises the steps of sending a training signal by said transmission circuit, sending a data signal by said transmission circuit, said data signal at least comprising said data modulated by said transmission circuit with orthogonal frequency division multiplexing, determining in said receiving circuit control information while receiving said training signal and from said training signal by using first time domain operations and demodulating in said receiving circuit said modulated data while receiving said data signal. Said training signal is at least partly a non-orthogonal frequency division multiplexing signal, for instance part of said training signal can be a Pseudo Noise sequence.
Said transmitting apparatus and said receiving apparatus can be said apparatus of the first aspect of the invention although the method is not limited thereto.